Physics of the Aurora

Abstract

The aurora, fascinating by its beauty and its multitude of forms, has turned out to be even more fascinating in terms of the physics to be learned from scientific study of it. In situ observations of the aurora and related phenomena have brought to light plasma physical processes, whose existence has a profound impact on our concept of space around us. Most of these processes are related to the auroral acceleration process. Whereas it has long been known that the aurora is caused by electrons of a few keY energy impinging on the upper atmosphere, the way in which these electrons gain their energy has been a crucial as well as controversial issue. There is now almost universal agreement that magnetic-field aligned electric fields play a key role, in confirmation of a prediction by Hannes Alfven more than three decades ago. Three main mechanisms that make such fields possible have been recognized. Probably all of them operate in the auroral acceleration region, but their relative roles are still to be determined. It has also become clear that there is an intric;ate interplay between these fields and various forms of wave-particle interactions involving time-dependent electric fields over a wide range of frequencies. Magnetic-field aligned electric fields have important consequences for the behaviour of a plasma, not only in terms of its capability to energize charged particles but also for the dynamics of the plasma itself, e.g. by violation of the "frozen field condition". Therefore the understanding of such fields also forms an important basis for understanding cosmical plasmas in general. The same forces that hurl the auroral electrons downwards also expel positive ions upwards into the magnetosphere. This expulsion can be so copious that occasionally large parts of the magnetosphere are dominated by oxygen plasma from the Earth's own ionosphere, rather than by hydrogen plasma from the solar wind. For reasons that we are only beginning to understand, the expulsion is highly selective. In other words, it constitutes an efficient chemical separation mechanism, whose very existence was completely unexpected until recently. As similar separation mechanisms may operate in other astrophysical plasmas, the significance of this discovery could be far-reaching. It has been emphasized by Alfven that the lessons learned in accessible regions of the space plasma necessitate a change of paradigm, which affects all of astrophysics, cosmology and cosmogony. Most of these lessons have c(,)me from study of problems related to the .aurora, and there may still be more to come.